"Kalian di sini bukan untuk bermain-main!"

Translation: You are not here to play around!

[BinginClare]

A correct English translation actually would be 'you are not here to play around ' Duo, you need more native English speakers to help you with sentences which actually make sense in English.

[WayangOrang]

Kalian di sini untuk belajar Bahasa Indonesia.

[CandyGill1]

It is more natural to say You're not here to play around not your are here not to play around, which doesn't make as much sense

[sabrinyxx]

The way she pronounced "main" in the end was unnatural. It is actually "ma-een", not the English "main".

[Ariatri]

Riteeee he's clearly not native indonesian

[YohannesSusilo]

The speaker has improved. I think they sound like some of the soap opera dubbers here, now.

[UpzahwvW]

Why is the negator "bukan" used for this sentence rather than "tidak" used? I'm trying to understand proper grammar patterns. Is it because the thing being negated is a prepositional phrase (or starting with "untuk" at least)?

[WayangOrang]

Is it because the thing being negated is a prepositional phrase

Yes.
"tidak" is used to negate verbs/adjectives/entire sentence.

Please have a look here:
Negation: Tidak, bukan, jangan, belum, Tips & Notes, Addendum.
https://forum.duolingo.com/comment/31291121

[Flanders1]

That is really bad sounding English. (native speaker)

[deactivated user]

"main" should have been sounded like "ma" from "mama" + "in" from "internet"

[wolfe.mcbr]

Is this sentence meaning youre here to work, not here to muck around and goof off? I dont like the english turn of phrase 'play around'.

[UpzahwvW]

It could mean that you are here to work (depends on context). It definitely means that you are not there to muck around and goof off (or not take seriously why you are there, nor do the expected task in a haphazard, unattentive manner).